Black ich is treated with formalin baths, not a FW dip. Black ich spots are worms, not like white ich. The worms will not be effected by the short period of time spent in FW during a FW bath. That is also why black ich is not treated via hyposalinity. They can take a wide variety of fluctuations.
FW dips are stressful to the fish. They should not be performed as a precaution. They should be performed ONLY as a last resort. There are much better ways to treat ich. Only use it if your fish is destined to die before other treatments can take effect.
If you all have questions about dips then please start your own threads describing your individual situation so that we can help you all out.